Now, after more than a decade as an F.B.I. agent, he is back at the S.E.C.

Mr. Makol, 44, rejoined the S.E.C. last month as a forensic accountant in its market-abuse unit, according to agency officials. As he did at the F.B.I., where he was known to warn suspects that he “would be watching,” Mr. Makol will investigate insider trading.

His move is an unusual twist on the so-called revolving door. While the typical path through that metaphorical portal leads to lucrative jobs at law firms and auditors, Mr. Makol has instead jumped from one federal agency to another.

Or, as one former colleague put it, rather than cashing out, “he gets to stay on Team America.”

Still, the salary might have been a draw to the S.E.C. Although the thought of joining a government agency in partfor money might sound laughable, a senior F.B.I. agent’s salary maxes out around $150,000, while the S.E.C. can pay more than $200,000. And unlike at the F.B.I., Mr. Makol, who has a 7-year-old son, will now rarely have to travel from his home base of Boston.

Mr. Makol, who grew up in Springfield, Mass., as the son of a forklift operator, has arrived at a crucial period for the S.E.C. market-abuse unit. The unit, run by the S.E.C. veteran Daniel M. Hawke, is tasked with investigating some of the most prominent yet least understood corners of the financial markets, including high-frequency trading and the private stock trading systems known as dark pools.

The unit, which recently filed cases against the Swiss banking giant UBS and the trading platform Direct Edge, worked alongside the F.B.I. and federal prosecutors on the insider trading crackdown that swept through Wall Street the last few years.

“David Makol had an outstanding career with the F.B.I., working in parallel with us on pathbreaking insider-trading cases,” Andrew J. Ceresney, the agency’s enforcement chief, said in a statement. “We are delighted that he has decided to rejoin us.”

Mr. Makol declined to comment.

According to his colleagues, Mr. Makol once pursued a career in finance, gaining experience at Fidelity. He then earned an M.B.A.

In the early 2000s, after roughly two years at the S.E.C., he joined the F.B.I. in New York, where he helped drive the insider trading inquiries.

Mr. Makol, known to arrive at work at 4:30 a.m., specialized in the art of the flip, coaxing suspects into cooperating against their colleagues. It became a central component of the insider trading investigations that led to more than 80 convictions.

Mr. Makol is credited with turning David Slaine, a former trader at the hedge fund Galleon Group, into a cooperating witness. Mr. Slaine provided critical evidence that enabled federal authorities to place wiretaps on phones used by the Galleon co-founder Raj Rajaratnam, who was eventually convicted and sentenced to 11 years in prison.

“He’d watch them from the minute they’d put their kids on the school bus,” recalled David A. Chaves, the F.B.I. agent who supervised Mr. Makol. “He’d follow them on the train on their way to work. He was relentless at understanding these people, probably better than they understood themselves.”

But not every suspect flipped, and some defense lawyers complained that the F.B.I. would pounce at their clients’ most vulnerable moments.

F.B.I. overtures to John Kinnucan, who ran Broadband Research, backfired when he sent an email to clients alerting them to an investigation. Mr. Kinnucan’s email, in which he mocked Mr. Makol and another agent as “eager beavers,” effectively halted certain undercover investigations. (Despite his taunts, Mr. Kinnucan eventually pleaded guilty.)

And Jonathan Hollander balked when Mr. Makol tried to turn him against his former employer, the hedge fund SAC Capital Advisors. It was not for a lack of trying. After confronting Mr. Hollander outside an Equinox gym, people briefed on the matter said, F.B.I. agents showed him a sheet of paper with pictures of employees at SAC, which the agents likened to a criminal enterprise.

Eventually, as the insider trading investigation waned, Mr. Makol transferred to the F.B.I. office in Boston, where he continued to work insider cases. He also led a team of investigators searching for suspects in the Boston Marathon bombing.

And even now that he has joined the S.E.C., a token of his F.B.I. cases remains: Some of his insider trading cooperators still send him holiday cards.

A version of this article appears in print on , on Page B3 of the New York edition with the headline: Foe of Inside Trading at F.B.I. Joins S.E.C.. Order Reprints | Today’s Paper | Subscribe

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